In the past three decades, electricity has risen from 25% to 40% of end-use energy consumption in the United States. With this rising demand for power comes an increasingly critical requirement for highly reliable, high quality power. As power demands continue to grow, older urban electric power systems in particular are being pushed to the limit of performance, requiring new solutions.
Wire forms the basic building block of the world's electric power system, including transformers, transmission and distribution systems, and motors. The discovery of revolutionary HTS compounds in 1986 led to the development of a radically new type of wire for the power industry; this discovery is the most fundamental advance in wire technology in more than a century.
HTS wire offers best-in-class performance, carrying over one hundred times more current than do conventional copper and aluminum conductors of the same physical dimension. The superior power density of HTS wire will enable a new generation of power industry technologies. It offers major size, weight, and efficiency benefits. HTS technologies will drive down costs and increase the capacity and reliability of electric power systems in a variety of ways.
For example, HTS wire is capable of transmitting two to five times more power through existing rights of way. This new cable will offer a powerful tool to improve the performance of power grids while reducing their environmental footprint.
However, to date only short samples of the HTS tape used in the manufacture of next-generation HTS wires have been fabricated at high performance levels. In order for HTS technology to become commercially viable for use in the power generation and distribution industry, it will be necessary to develop techniques for continuous, high-throughput production of HTS tape.
MOCVD is a deposition process that shows promise for the high throughput necessary to cost-effectively produce HTS tapes. During MOCVD, HTS film, such as yttrium-barium-copper-oxide (YBa2Cu3O7 or “YBCO”), may be deposited by vapor-phase precursors onto a heated buffered metal substrate via chemical reactions that occur at the surface of the substrate.
One way to characterize coated conductors is by their cost per meter. Alternatively, cost and performance can be characterized as the cost per kiloamp-meter. More specifically, by increasing the current for a given cost per meter of coated conductor, the cost per kiloamp-meter is reduced. This is evidenced in the critical current (Jc) of the deposited HTS material multiplied by the cross-sectional area of the film.
For a given critical current and width of coated conductor, one way to increase the cross-sectional area is by increasing the HTS film thickness. However, under conventional process parameters it has been demonstrated that with critical current as a function of thickness, the critical current drops off as the thickness of a single layer of HTS film increases beyond approximately 1.5 microns and may reach saturation. This is because beyond a film thickness of approximately 1.5 microns, the HTS material becomes very porous, develops voids, and develops increased surface roughness, all of which contribute to inhibiting the flow of current. This results in limiting the critical current in coated conductors to typically 100 A/cm width.
Since, under conventional process parameters simply increasing the HTS film thickness does not result in a corresponding increase in critical current, a technical challenge exists to increase the HTS film thickness above 1.5 microns and at the same time realize a corresponding increase in current density.
In an MOCVD deposition process, factors that contribute to the morphology of the HTS film include the chamber pressure, the substrate temperature, the oxygen content and its method of introduction to the deposition zone, the amount of precursors being supplied to the deposition zone (determined by both the precursor molarity and the mass flow rate of the precursor vapors and their inert carrier gas through the showerhead assembly), the temperature at which the precursors are maintained prior to their introduction into the deposition zone, and the exhaust efficiency of the reaction byproducts away from the deposition zone.
While the optimization of some of the aforementioned parameters is well known, such as the fact that the precursor vapors and their inert carrier gas are most efficiently delivered to the deposition zone within a temperature range of 230 to 270° C., the optimization of other parameters is less well known, requiring technical innovations to be realized.
Hubert, et al., U.S. Pat. No. 5,820,678, entitled “Solid Source MOCVD System,” describes a system for MOCVD fabrication of superconducting and non-superconducting oxide films that provides a delivery system for the feeding of metalorganic precursors for multi-component chemical vapor deposition. The delivery system can include multiple cartridges containing tightly packed precursor materials. The contents of each cartridge can be ground at a desired rate and fed together with precursor materials from other cartridges to a vaporization zone and then to a reaction zone within a deposition chamber for thin film deposition. A drawback of the MOCVD system of Hubert, et al., is that while it is suitable for depositing superconducting oxide films, it does not provide a process for increasing the critical current of thick HTS films.
Tatekawa, et al., U.S. Pat. No. 6,143,697, entitled “Method for Producing Superconducting Thick Film,” describes a method of producing a superconducting thick film that involves the steps of forming a thick layer comprising a superconducting material on a substrate, firing the thick layer formed on the substrate, subjecting the fired thick layer to cold isostatic pressing, and re-firing the thick layer subjected to cold isostatic pressing. A drawback of Tatekawa, et al., is that while the method is suitable for forming superconducting oxide thick films, it is not a cost-effective way of manufacturing high-current HTS-coated conductors nor does it provide controlled process parameters sufficient to produce thick HTS films with increased critical current. Tatekawa, et al., is therefore not suited for the cost-effective production of high-current HTS coated conductors.
It is therefore an object of this invention to produce a high-current HTS-coated conductor with coatings formed by HTS film with a thickness in excess of 1.5 microns that have increased current capacity, over 100 A/cm width.
It is another object of this invention to provide a low-cost method of forming high-current HTS-coated conductors using an MOCVD process for producing multiple layer HTS coated tapes.
It is an object of this invention to produce YBCO films with a thickness in excess of 1.5 microns with increased current capacity for use in the manufacture of high-current HTS-coated tape.
It is an object of this invention to provide a cost-effective method of forming high-current HTS-coated conductors using an MOCVD process with precisely controlled process parameters for the deposition of YBCO thick films.
It is an object of this invention to provide a cost-effective method of forming high-current multi-layered HTS-coated conductors where the multi-layers have the same composition. It is an object of this invention to provide a cost-effective method of forming high-current multi-layered HTS-coated conductors where the multi-layers have different compositions.